Pigmented fluorocarbon coatings are recognized today for the coating of architectural panels, it being intended that these coatings provide an attractive finish which will resist fading and chalking for long periods of time. The attractive finish demands that the coatings be heavily pigmented to obliterate sight of the substrate or primer coat with pigments which do not well resist weathering and the ultraviolet radiations contained in sunlight. As a result, and as taught in U.S. Pat. No. 4,314,004 issued Feb. 2, 1982 to Richard L. Stoneberg, the heavily pigmented coatings are not directly exposed to the elements, but are instead overcoated with a clear fluorocarbon topcoat which protects the pigmented basecoat against the elements and screens out some of the sunlight so that the pigmented coating layer does not change color or fade. Unfortunately, these clear topcoats introduce their own problems, namely: the topcoats lack solvent resistance and film integrity and the fluorocarbon resin in the topcoat tends to yellow upon baking. The solvent resistance of topcoats has previously been addressed in commonly owned U.S. Pat. No. 4,659,768 issued Apr. 21, 1987 to Anthony J. Tortorello and Clark A. Higginbotham, but the inadequate color stability and yellowing tendency on baking of the clear topcoats have not hitherto been resolved.
This industrial problem has persisted for several years, the workers in the art being unable to overcome it. On the one hand, the pigmented coatings can be left unprotected, in which case the pigments therein fade and the initial color changes with time. On the other hand, the pigmented coatings can be overcoated with a clear fluorocarbon topcoat, in which case the clear topcoat has poor mar resistance and tends to yellow with baking (the thicker the coating, the more the yellowing).